LIBRARY ı6ıul 0) - Cranfield University

" to design and build the hardware necessary for on-line monitoring and
control;
" to develop a sensor for pre-weld position adjustment;
" to develop monitoring algorithms for metal transfer, process stability and
stand-off,
" to develop a robot independent part positioning system giving precise
stand-off control.
8.2 Robot off-line programming
8.2.1 Analysis of sources of error
The analysis of the possible sources of error and corresponding corrective
actions have shown that their detrimental effect could be reduced or eliminated (see
Chapter 3). For a better understanding of the nature of each possible source of error,
the errors identified were classified into three main groups, namely: (a) Robot errors,
(b) Programming errors and (c) Component errors.
The robot error group included those errors caused mainly by the positioning
hardware, including the robot itself and the positioning table, when present. The
programming errors included mainly the inaccuracy in the geometrical models used to
represent the robot and its environment in the computer "virtual world". They also
included the errors caused by using an inadequate inverse kinematics algorithm to
calculate the robot joint angles. Although very different in nature, these two error
groups presented the similarity of having a means to reduce or even eliminate their
detrimental effect. The robot errors could be greatly reduced by using robot
calibration techniques [ref. 94]. The programming errors could also be reduced by
correcting the robot model using the parameters obtained from the calibration
procedures and the robot environment model using the positional data which could be
obtained utilising the calibrated robot as a measuring tool [ref. 88,92].
The component error group included the errors due to part, joint fit-up and
fixturing tolerances and also due to wire cast, contact-tip wear, thermal distortion and
part positioning. Despite not being all originated at the components themselves, these
errors often affect the relative positioning between the tip of the welding wire and the
joint line and also the geometry of the joint in the form of gap and misalignment. Their
detrimental effect have the similarity of being variable and not always possible to
predict. Therefore, some form of process and relative position monitoring and control
is needed in order to minimise or eliminate their detrimental effect on weld quality.
Since robot calibration techniques are relatively well established, existing
methodology [refs. 94], such as the one implemented using the RoboTrak system [ref.
97], could be used to correct for the robot and the programming errors. Hence, this
present work has concentrated only on compensating for the errors originated in the
component error group.
188